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Ch 16 Evolution of populations. Crash Course: Population Genetics. https://www.youtube.com/watch?v=WhFKPaRnTdQ. 16-1 Genetic equilibrium. Population genetics : study of evolution from a genetic point of view Basically how populations of a species evolve But what is a population? - PowerPoint PPT Presentation
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CH 16EVOLUTION OF POPULATIONS
Crash Course: Population Genetics https://www.youtube.com/watch?v=WhF
KPaRnTdQ
16-1 Genetic equilibrium Population genetics: study of
evolution from a genetic point of view Basically how populations of a species
evolve But what is a population? Group of members of the same species
living in the same area
Sources of genetic variation Three main sources1. Mutations: any change in sequence of
DNA Replication mistakes Radiation/environmental causes
2. recombination: reshuffling of genes3. Random pairing of gametes
Bell curve Many traits in nature show trends like
this
Phenotype continuum
# of
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als w
ith th
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ait
Number of phenotypes produced depends on how many genes control that trait
Single gene traits- have two alleles Two distinct phenotypes
Polygenic traits- controlled by two or more genes
Results in multiple phenotypes
Gene pool- all genes, including all different alleles, that are present in a population
frequency (of an allele)- number of times alleles occur in a gene pool Percentage
Genetic definition of evolution? Change in relative frequency of alleles in
a population over time
Phenotype frequency How often a specific phenotype is
observed in a population Can be written mathematically
Frequency = # indiv. w/a particular phenotype
total # of indiv. in population
Hardy-Weinberg equilibrium When evolution is not occurring
Allele frequencies remain the same In order for evolution to not occur,
certain conditions must be met.
Evolution Versus Genetic Equilibrium
Hardy-Weinberg principle = Genetic Equilibrium• Random Mating – Equal opportunity to produce
offspring• Large Population – Genetic Drift does not effect
Allele Frequency• No Movement into or out of Population – The
gene pool must be kept together (no new alleles)• No Mutations – Mutations cause new forms of
alleles changing the frequency• No Natural Selection – All genotypes must have
equal probability of surviving.
Hardy-Weinberg equilibriumAllele frequency equation p + q = 1 p = frequency of dominant allele q = frequency of recessive alleleTogether, they make 100% of alleles for a gene in that population If p = 34%, what is q? If q = 19%, what is p?
0.660.81
Hardy-Weinberg equilibrium Genotypic frequency equation
p2 + 2pq + q2 = 1 p2 = homozygous dominant frequency 2pq = heterozygous frequency q2 = homozygous recessive frequency If p = .46, what is p2? If p = .12, what is q2? If q =.31, what is 2pq?
0.21160.7744 = 77%0.4278
16-2 Disruption of genetic equilibrium
Mutation Occur at a relatively constant rate over
time Can be sped up when exposed to mutagens
Gene flow: process of genes moving from one population to another
Immigration: moving into a population Emigration: moving out of a population
Genetic Drift Alleles can become rare by chance Over time a series of chance
occurrences can cause an alleles to become common in a population
Effects of genetic drift are more dramatic with small population size
Founder effect: change in allele frequencies as a result of migration of a small subgroup of a population
Sample of Original Population
Founding Population A
Founding Population B
Descendants
Genetic DriftSection 16-2
Sample of Original Population
Founding Population A
Founding Population B
Descendants
Genetic DriftSection 16-2
Sample of Original Population
Founding Population A
Founding Population B
Descendants
Genetic DriftSection 16-2
Nonrandom mating Sexual selection: tendency of
individuals to choose a mate with certain traits.
Common in birds Peacock display Tropical birds of paradise - Papua New G
uinea The amazing Lyrebird - Australia
Natural selection Natural selection on a single gene traits
can lead to changes in allele frequencies Natural selection on polygenic traits
3 possible effects1. Directional selection2. Stabilizing selection3. Disruptive selectionhttps://www.youtube.com/watch?v=vCHdT9MWIaA
Directional selection When individuals at one end of curve
have higher fitness than individuals in the middle or the other end
Stabilizing selection When individuals near the middle have
higher fitness than the individuals at either end
Disruptive selection When individuals at upper and lower
ends have higher fitness than individuals near the middle
16-3 Formation of Species As new species evolve, populations
become reproductively isolated from each other
Reproductive isolation: when two members of populations cannot interbreed and produce fertile offspring
Separate gene pools
Isolation Mechanisms
Geographic Isolation:- separation of animals in a
specific region- formation of river, canyon,
mountain
Isolation Mechanisms
Behavioral Isolation:- differences in courtship or reproductive behaviors-meadowlark songsTemporal isolation:
-two or more species reproduce at different times
-orchids
Formation of species Allopatric
speciation: when species arise from geographic isolation Different places
https://www.youtube.com/watch?v=cSgulsydsQU
Reproductive isolation Prezygotic isolation:
premating isolation Species may live
in different places Reproduce at different times Have different mating
behaviors Postzygotic isolation:
postmating isolation Hybrids may be weak Hybrids may be sterile
Sympatric speciation
Sympatric speciation: when two subpopulations become isolated while living in the same area
Rates of speciation Gradualism:
speciation at gradual and regular rate
Punctuated equilibrium: periods of sudden, rapid change followed by periods of littelchange